German patent document DE 198 52 399 A1 discusses a braking value encoder, which can be activated by a foot-operated brake pedal, for an electronic brake system, which braking value encoder generates a pneumatic brake pressure in a pneumatic channel as a function of the activation, and an electrical braking request signal in an electrical channel via an electrical sensor. In the event of electrics or electronics failure, the pneumatic brake pressure which is generated in the pneumatic channel by the braking value encoder becomes effective as the brake pressure.
As discussed in DE 10 2006 029 349 A1, the closed-loop control arrangement for a brake system with an ABS functionality contains a wheel pressure closed-loop controller which is controlled by a superordinate function which calculates the pressure requirements. The wheel pressure closed-loop controller then sets the desired wheel pressure. The control quality of an ABS closed-loop control arrangement depends here substantially on the closed-loop control accuracy of the wheel pressure closed-loop controller. If the pressure requirements are implemented imprecisely, the ABS closed-loop control arrangement cannot keep the respective wheel in the optimum slip range. As a result, adverse effects incur on the braking power or stability during braking.
Optimum traction control can be achieved if separate pressure sensors are present in each of the wheel brake circuits. However, for the sake of cost-effective fabrication of the brake control system it is often the case that no wheel pressure sensors are present with the result that the current wheel pressure which is necessary for the closed-loop control arrangement has to be determined by a wheel pressure model. However, in order to determine the wheel pressure by the wheel pressure model, at least one item of information about the admission pressure which is present is required in order to be able to infer the current wheel pressure from actuation times of the inlet and outlet valves. Consequently, the wheel pressure controller in which the wheel pressure model is implemented uses the admission pressure measured with an admission pressure sensor at the braking value encoder as an output value for determining the current wheel pressure.
FIG. 1 shows an ABS closed-loop control arrangement for pneumatic or electro-pneumatic brake systems according to the prior art, in which ABS closed-loop control arrangement, when a braking operation is initiated by activating a braking value encoder 6, the wheel rotational speed signals which are supplied by wheel rotational speed sensors 1 are evaluated in a control unit 2 and the respective brake slip of the wheels which are provided with wheel brake cylinders 4 is calculated. The ABS closed-loop control arrangement, which are implemented in the control unit 2, then adjust, via ABS valves 8, the brake slips of the wheels via ABS control valves 8 to an optimum slip control range in order to achieve the shortest possible braking distance and stable vehicle behavior.
On the basis of the limited supply pressure in the reservoir containers and the relatively long pneumatic piping, in particular in utility vehicles, such ABS closed-loop control is possible without information about the admission pressure at the ABS control valve 8, but when the admission pressure information is dispensed with the ABS closed-loop control is imprecise and the braking distance becomes longer and the ABS closed-loop control becomes uncomfortable. The ABS closed-loop control becomes more comfortable and more precise compared to this if a piece of information about the admission pressure such as can be determined using an admission pressure sensors which is arranged upstream of the ABS control valve 8 is present in the ABS system.
This admission pressure sensor 5 measures the admission pressure in a feedline of the ABS control valve 8 which then modulates a brake-slip-optimized brake pressure at the brake cylinder of the assigned wheel 4. The actuation times of the ABS valves 8 can be calculated on the basis of known characteristics of the ABS control valve 8 and the information supplied by the admission pressure sensor 5 by the admission pressure, in order to obtain the respective target pressure at the brake-slip-controlled wheel 4. An admission pressure sensor 5 is therefore understood to be a pressure sensor which measures the brake pressure which is present upstream of an ABS control valve 8 and is therefore connected upstream of the ABS control valve.
However, such ABS admission pressure sensors are associated with certain costs. Furthermore, failure of admission pressure sensors or of the evaluation unit thereof at the same time brings about a failure of the ABS closed-loop control by the ABS control valves.
Furthermore, in the case of an electronically closed-loop controlled breaking device (EBS) in the case of pressure closed-loop control modules (DRM) which are switched off as a function of faults, the ABS closed-loop control arrangement can no longer take place by the ABS valves which are connected downstream of the pressure closed-loop control modules, since the pressure sensors which are integrated in the pressure closed-loop control modules can then no longer supply any information about the ABS admission pressure.